The present invention relates to a resonance type non-contact power transmission apparatus.
FIG. 5 schematically shows a resonance type non-contact power transmission apparatus that transmits power from a first copper wire coil 51 to a second copper wire coil 52 placed at a distance from the first copper wire coil 51 via resonance of electromagnetic fields. Such an apparatus is disclosed, for example, in NIKKEI ELECTRONICS published on Dec. 3, 2007, pages 117 to 128 and International Patent Publication No. WO/2007/008646. In FIG. 5, a magnetic field generated at a primary coil 54 connected to an AC power source 53 is intensified via magnetic field resonance of the first and second copper wire coils 51, 52. The effect of electromagnetic induction from the intensified magnetic field around the second copper wire coil 52 generates power in the secondary coil 55. The generated power is then supplied to a load 56. It has been observed that a 60-watt electric lamp, as the load 56, can be lit when first and second copper wire coils 51, 52 having a diameter of 30 cm are separated by 2 m.
To effectively supply output power of the AC power source 53 to the load 56 in this resonance type non-contact power transmission apparatus, it is necessary to efficiently supply the output power of the AC power source 53 to a resonance system (the first and second copper wire coils 51, 52 and the primary and secondary coils 54, 55). However, the above cited documents do not specifically show what should be done to obtain a resonance type non-contact power transmission apparatus that efficiently supplies output power from the AC power source 53 to a resonance system.
When the distance between the first copper wire coil 51 and the second copper wire coil 52 and the impedance of the load 56 are constant, the resonant frequency of the resonance system is obtained in advance by experimentation. An AC voltage having the obtained resonant frequency is supplied from the AC power source 53 to the primary coil 54. However, if at least one of the distance between the first copper wire coil 51 and the second copper wire coil 52 and the impedance of the load 56 changes, the input impedance of the resonance system at the resonant frequency changes. Thus, the output impedance of the AC power source 53 and the input impedance of the resonance system do not match. This increases the reflected power from the resonance system to the AC power source 53, and therefore hinders efficient supply of output power from the AC power source 53 to the load 56. In this description, the resonant frequency of the resonance system refers to the frequency at which the power transmission efficiency η is maximized.
For example, when the resonance type non-contact power transmission apparatus is used for charging a battery, the load of the battery changes according to the battery charge state. This changes the input impedance of the resonance system and thus increases the reflected power to the AC power source.